1. Field of the Disclosure
The disclosure relates to a chuck, in particular for use in a mask aligner for aligning a wafer in parallel to a mask and a method for setting a gap between a wafer on a top plate of a wafer chuck and a mask.
2. Discussion of the Background Art
In the following, the information is frequently described with reference to the alignment of a wafer with respect to a mask. This reference to a wafer and a mask, however, should not be construed as a limitation, i.e. the claimed chuck and the method for setting a gap are equally applicable to the alignment of a first and second planar substrate in general.
For the production of microelectronic, micro-optical or micromechanical elements and devices, it is a common process to transfer a structure from a mask onto a substrate, e.g. a silicon wafer, using photolithography. The photolithography uses light to transfer said structures, e.g. geometric patterns, from the mask to a photo-sensitive chemical resist which is coated onto the substrate before. Chemical agents then etch the exposed structures in the substrate or enable deposition of other material in the transferred patterns onto the substrate. Industrial production may require the repetition of this process up to 50 times.
If the mask and the substrate are not in parallel, i.e. if there is a wedge between the mask and the wafer, the structures of the mask cannot be transferred uniformly onto the substrate resulting in defects and improper structures, in general in a reduced process window for the customer. In view of the numerous repetitions of the photolithography process, such a wedge error can lead to a high loss of production. Consequently, a proper alignment of the wafer and the mask with respect to each other is necessary. If the mask and the substrate are not repeatable positioned at the proper distance, structures will not be transferred accurately to the substrate.
As modem production processes use increasingly large substrates, the aforementioned alignment becomes increasingly challenging. The compensation of said wedge error is typically performed using a wedge error compensation (WEC) head.
Such a WEC head uses a passive mechanical system to align the wafer top surface with the mask bottom surface. The WEC head comprises a bottom part that is fixed and a top part that is movably supported and interconnected to the bottom part by means of springs. During the WEC process, the wafer and the mask are brought into mechanical contact (e.g. through high precision distance balls), subsequently the top is fixed against the bottom part in this position by means of brakes. As a result, the wafer and the mask are aligned in parallel.
However, for large substrates as well as sub-micron structures, such an alignment is not sufficiently precise. WO 2011/098604 A2 relates to a device for active WEC wherein the afore-mentioned known WEC head is complemented with piezoelectric actuators to allow for a more fine alignment on the basis of mechanical sensors or caliper; this method, however, allows a relative measurement during the WEC process only and requires a mechanical contact for each separate wafer. In addition, said method is less suitable for automatic machines due to relatively high abrasion.
US 2002/063221 A1 relates to a gap adjustment apparatus and gap adjustment method for adjusting gap between two objects.
EP 0 722 122 A1 relates to a process and device for adjusting the distance between a workpiece and a mask.
Further background art is listed as follows: JP 2004 233398 A, JP 11 194501 A, US 2007/190638 A1, EP 2 006 899 A2, JP 3 038024 A, JP 2008 310249 A, JP 58 103136 A.
It is an object of the disclosure to provide a device and a method to position two planar surfaces, in particular a wafer top surface and a mask, parallel to each other. In particular, it is an object of the present disclosure to provide a chuck, e.g. a wafer chuck, for use in combination with a WEC head or a chuck which even replaces a WEC head of a mask aligner for aligning a wafer in parallel to a mask. Furthermore, it is an object of the disclosure to provide a method for setting a gap between a wafer on a top plate of a wafer chuck and the mask.